In conventional centrifugal pumps of the volute type, the section of the pump casing surrounding the periphery of the impeller is of changing cross-section. The outer peripheral profile is made to approximate a volute shape having a radius of curvature increasing to a maximum at a point where it becomes tangential to a discharge nozzle. Not only does the cross-sectional area of this volute section of the casing vary but the cross-sectional profile also varies around the periphery of the pump. The normal volute type casing therefore has a complex shape.
Centrifugal pumps are often fitted with replaceable abrasion resistant liners, especially pumps for pumping slurries. Refer, for example, to U.S. Pat. Nos. 4,243,291 to Hurst et al, "Wear Lining", and 4,264,273 to Grzina, "Casing and Casing Liners for Centrifugal Pumps of the Volute Type", the disclosures of which are herein incorporated by reference. These well-known liners generally have contours which essentially correspond to the contours of the pump casings into which they will be inserted.
Known also are casing liners having uniquely contoured interior surfaces which may or may not correspond to the interior wall configuration of the pump casing. Refer, for example, to U.S. Pat. No. 3,265,002 to Warman, "Centrifugal Pumps and the Like", the disclosure of which is herein incorporated by reference. The disclosure of Warman refers to obtaining gains in pump performance by controlling the shapes of the hydraulic passages in the volute region.
Regions of instability in pump performance profiles, where fluid flow through the pump becomes unstable, are well-known. Unstable flow through a pump is defined as an abrupt change in pressure or efficiency. A cyclic pattern of flow and pressure swings could trigger surging or vibration which is known to be damaging to both the pump and the system. Traditionally, high specific speed pumps and fans are characterized by an inherently unstable flow at low flow rates. The mechanism causing the instability in these cases is thought to be due to flow streamlines stalling or separating at the impeller inlet vanes. This condition is acknowledged and generally accepted in the industry such that pump or fan operation in such unstable zones is generally avoided.
In centrifugal pumps for pumping slurries, the unstable flow conditions can result from other mechanisms and/or parameters, such as "distorted", i.e., unusually wide (compared to the width of the impeller discharge opening) volute hydraulic passages. Slurry pumps typically have very wide impellers dictated by low velocity designs so as to minimize wear and provide the required thick shrouds to allow space for expellers or allowances for sacrificial wear. In slurry pumps, the combined thicknesses of the impeller shrouds adjacent the impeller discharge opening at the outer periphery of the impeller is typically at least approximately one-third to one-half the width of the recirculation zone at the outer periphery of the impeller. By contrast, the combined thicknesses of the impeller shrouds adjacent the impeller discharge opening at the outer periphery of the impeller of a clear water pump is a far smaller proportion of the width of the recirculation zone at the outer periphery of the impeller (i.e., typically only a maximum of about 0.14 the width of the recirculation zone) since clear water pump impellers have no sacrificial material applied thereto. The larger thicknesses of the impeller shrouds in slurry pumps causes an abrupt increase in cross-sectional flow area (i.e., approximately 50% or more) as the slurry flows radially outwardly from the impeller to the collector region of the volute and thus creates turbulent flow patterns not present in clear water pumps. In prior slurry pumps, such flow turbulence causes instabilities and inefficiencies. The increased thicknesses of the impeller shrouds in prior slurry pumps also results in the volute having a marked non-circular cross-sectional shape which prevents a smooth transition between the volute and discharge nozzle in conventional slurry pumps.
In the case of slurry pumps without expellers or with worn expellers, unstable flow has been found to occur closer to design point than is the case for clear water pumps. Aside from destructive surging or vibration, unstable flow in a slurry pump is known to accelerate wear due to the dissipation of energy. A sudden drop in pressure and efficiency is an index of this dissipation of energy. The loss in static pressure is believed to be due to turbulence or destructive high velocity vorticies, which occur in the zone of instability.